1. Field of the Invention
This invention relates to anisotropic compounds and more particularly to liquid crystal mixtures (LC-mixtures) which contain these compounds and are useful as dielectrics in liquid crystal apparatus.
2. Description of the Prior Art
The best known LC-compounds correspond to the following formulae (10) and (11): ##STR4## wherein the rings K, and L, are optionally M and benzene, transcyclohexane or heterocyclic rings, X' and optionally Y' are direct bonds and/or carboxy groups and/or azomethine groups, and R' and R" are alkyl, alkoxy, alkylamino, alkylcarbonyloxy or nitrile groups.
Representative publications describing the current state of the art are German Offenlegungsschriften No. 3,306,738; 2,306,739; 2,545,121; 2,636,684; and 2,701,591, as well as the corresponding disclosures in "Liquid Crystals in Tables", by D. Demus et al, Leipzig, 1974.
In the case of LC electrooptic twist cells, it is generally recognized that a nematic LC-mixture with a sufficiently high positive dielectric anisotropy is suitable for use as a dielectric. Molecules having a large dielectric anisotropy can be obtained by incorporating substituents having large dipole moments parallel to the long axis of the molecule into the compounds of formulae (10) and (11). The dielectric anisotropy is defined as the difference between the dipole moment normal to the molecular axis and that parallel thereto, that is EQU .DELTA..epsilon.=.epsilon..sub..vertline..vertline. -.epsilon..sub..perp.
Typically in these compounds R' is an alkyl or alkoxy group, while R" is a nitrile, alkyl, or alkoxy group.
Because of this structure and the contribution of the substituents perpendicular to the molecular axis, the .epsilon..sub..perp. is about 5, whereby the .epsilon..sub..vertline..vertline., for example, can be about 5 or can vary between 20-30. This has been established by many authors. See, for example, L. Pohl et al, Phys. Lett. 60 A, 421 (1977); A. Boller et al, Mol. Cryst. Liq. Cryst. 42, 215 (1977); R. T. Klingbiel et al, J. Am. Soc. 96 (25), 7651 (1974).
The incorporation of a cyclohexane ring, in contrast to a benzene ring, in the structural formulae (10) and (11) has the advantage that the compounds have a lower viscosity. However, such compounds have the disadvantages that the tendency to form smectic phases is enhanced and the so-called .DELTA.n-value, which is determined analogously to .DELTA..epsilon. from the difference between the refractive indices perpendicular and parallel to axis of the molecule, is decreased.
Commercially useful LC-apparatus requires LC mixtures having specific and limited threshold voltages and specific positive .DELTA..epsilon. values, e.g., from 1 to 10 or greater. According to the present state of the art, these conditions can only be satisfied by using mixtures or components which have a large .DELTA..epsilon. value and those which have a very low .DELTA..epsilon. value, which again tends to favor the formation of smectic phases, as has been determined by several authors (G. Heppke et al, Z. Naturforsch. 33a, 185 (1978) and B. Engelen et al, Ann. Phys. (Paris) 3, 403 (1978)).
An increase of .epsilon..sub..perp. with constant .DELTA..epsilon. value which is required for the threshold voltage value is desirable (see, for example, A. R. Kmetz, SJD Digest, Techn. Papers IX, 70 (1978)).
This goal is unattainable with the hitherto known LC-compounds of the formulae (10) and (11), since the only known nematic LC-compounds with positive .DELTA..epsilon. values have .epsilon..sub..perp. values around 5. Hitherto the problem has been circumvented by mixing element LC-materials which have positive .DELTA..epsilon. values with components which have a negative .DELTA..epsilon. value. This again leads to a relatively high threshold voltage and often to the appearance of smectic phases.
Hence, a need has continued to exist for LC-compounds which combine a large .DELTA..epsilon. value with a suitable threshold voltage.